In the automotive industry the transfer of hot exhaust gases from the exhaust manifold in an engine compartment to a tail pipe is a problem because many of the operating components in the engine compartment in proximity to the exhaust system may be prematurely damaged or aged by excessive heat. High temperature operating systems which are not necessarily limited to the automotive industry face the same problem.
As a result of the existing problem, an industry has risen which provides concentric metal tubes or conduits of some length having a flange secured at one end to hold them in proper orientation. That is, the radially spaced tubes provide a heat insulating air gap between the two conduits. Unfortunately, the air gap is not adequate to maintain the outer tube at a suitable low temperature.
What is needed is a mechanism for filling the annulus between the concentric conduits with a material having a low coefficient of thermal conductivity to increase the insulation factor between the inner and outer conduits.
Techniques for forming concentric tubes from some material (usually steel) with an annular gap between the two conduits is a known technology. A patent to Wilkenloh, No. 4,104,481 teaches a technique for forming concentric tubes with a foamable resin in the gap between the tubes. The purpose of the resin filling the annulus between the two conduits is to create a dielectric shield between concentric conductors of electricity. The technique used to form the layers of strands of electrically conducting cable is to form the layers sequentially from the inside out.
A patent to Jarrin et al., No. 4,963,420 is a similar technique where there are a plurality of inner conduits or cables surrounded by an outer sheath. Again the technique for forming the composite cable is to form the inner conduits and feed them through a nozzle system where the insulation material is extruded in surrounding relationship and thereafter passing the composite conduit through another extrusion process where the outer conduit is formed.
The system described in relation to these two patents is certainly a suitable solution to some problems in some industries, but the particular problem in the existing system does not lend itself to this particular technique. The metallic conduits of the existing system are formed as a unit and have an integral flange at one end. The end of the concentric conduits remote from the flange is open. That is, the inner tube has a completely open passage suitable for the transmission of hot gasses or liquids. The gap or annulus between the concentric conduits is open at that one remote end only. The concentric conduits are structured to be welded or otherwise mechanically attached to some other system where the inner conduit will convey the hot gas or liquid from one end to the other. The air gap between the conduits is to serve as an insulation barrier to minimize the heat transfer from the hot fluid being conveyed to the outer conduit. In those environments where the temperature of the outer conduit rises to an undesirable level over time a solution must be found to shield other operating components in the vicinity of the outer conduit from heat radiation and/or conduction. Certainly one technique is to apply an insulation to the exterior of the outer conduit. The reason such a technique is not desirable is because the externally applied insulation layer may be scraped or displaced during assembly of the concentric conduits.